Prior to his viceregal appointment Onley was a television journalist. He worked primarily for Citytv as a weather reporter, before moving on to cover science and technology stories. Later on, he worked with the 24-hour news station CablePulse 24 as a news anchor and host of a weekly technology series, Home Page. A published author, he was founding president of the Aerospace Heritage Foundation of Canada.

His seven-year term makes him the longest serving Lieutenant Governor of Ontario since Albert Edward Matthews (1937-1946) and the province's second longest serving viceroy since Confederation.

Beginning at the age of three,[7] Onley battled with polio, resulting in partial paralysis. However, as a result of extensive physical therapy, he regained the use of his hands and arms, and partial use of his legs. Onley is able to walk using leg braces and canes or crutches, but he generally prefers to get around using his electric scooter. He is able to drive a car using hand controls for acceleration and braking.

Onley began his career in radio, hosting a weekly science show for Toronto radio station CFRB, subsequently joining the CKO network in 1983. He then joined Citytv in 1984 as weather specialist, a position he held until 1989. In a 2004 interview with Link Up, a Toronto employment agency for people with disabilities, Onley stated that

“

At the time I remember saying to my mother, ‘I don't know if I should take this job (at Citytv). I don’t know if they’re hiring me because I’m disabled.’ My mother said, ‘You’ve been turned down enough times because of your disability, so take it!’ I thought to myself, ‘Damn it, she’s right’ and that’s how my career at Citytv began.[8]

”

From 1989 to 1995, he was the first news anchor on the then-new Breakfast Television, Citytv's morning show. He served as education specialist for Citytv and CablePulse 24 from 1994 to 1999. Onley became an anchor on CP24, when the station launched in 1999, and both hosted and produced Home Page on CP24.[6]

He was one of Canada's first on-air television personalities with a visible disability;[6] he uses a mobility device due to his paralysis. Camera shots began with only upper body shots, but Onley demanded that the shot include him in his mobility device. In honour of his contributions to the advancement of disability issues in Canada, he has received awards from the Terry Fox Hall of Fame in 1997,[6] and the Clarke Institute's Courage to Come Back award.[6] He was appointed Chair of the Accessibility Standards Advisory Council to the Minister of Community and Social Services in 2005.[9] He was most recently inducted into the Scarborough Walk of Fame in 2006.

Onley wrote Shuttle: A Shattering Novel of Disaster in Space, a bestselling novel about space travel, published in 1981. It was nominated by the Periodical Distributors of Canada as book of the year. He was founding president of the Aerospace Heritage Foundation of Canada.

Onley returned to his acting roots, with a cameo appearance in the sixth season of the Canadian TV series Murdoch Mysteries. The episode, "The Ghost of Queens Park" aired in Canada on February 25, 2013. In it he played the eighth Lieutenant Governor of Ontario, Sir Oliver Mowat.[10]

Onley's appointment as Lieutenant Governor was announced on July 10, 2007; he was privately informed of this after a July 4, 2007, taping of Home Page: "I just had reached the top of the Don Valley Parkway... and there was no place to pull over. And when the Prime Minister of your country calls, all you can try to do is stay in the same lane, avoid any fender-benders and have a meaningful conversation, which I did."[4]

He was sworn in on September 5, 2007, at Queen's Park in Toronto. As the province's first Lieutenant Governor with a disability, Onley said he would use his vice-regal position to help remove physical barriers to Ontario's 1.5 million people with disabilities, as well as focus on other issues affecting the disabled, including obstacles to employment and housing. Onley also stated, in his installation speech, that he would expand on his immediate predecessor James Bartleman's First Nations literacy initiatives, his aim being to see computers on every student's desk in northern schools.[11] For his installation, Onley approached the legislature on his electric scooter, however he ascended the Throne on foot, using leg braces and canes. Onley also travelled to China to represent the Queen and Canada at the 2008 Summer Paralympics opening ceremony.[12] Onley delivered his last speech from the throne to the Ontario Legislative Assembly on July 3, 2014; his last full day in office was September 22, 2014 with his successor sworn in the following afternoon.

Onley and his wife resided in their Scarborough home during his vice-regal tenure as Ontario is one of three provinces that does not have an official vice-regal residence.[13]

During Onley's mandate, he participated in 2550 engagements, during which he spoke to an estimated audience of over one million people.[14]